Pulsed NMR techniques are used in instruments for the measurement of the type, property and quantity of lattice bound and free, magnetically active nuclei within a sample. Some of the substances and properties that have been measured by NMR techniques are: moisture, polymers and copolymers, oils, fats and crystalline materials.
Pulsed NMR uses a burst or pulse of energy that is designed to excite the nuclei of a particular nuclear species of a sample being measured (the protons, or the like, of such sample having first been precessed in an essentially static magnetic field); in other words the precession is modified by the pulse. After the application of the pulse there occurs a free induction decay (FID) of the magnetization associated with the excited nuclei. That is, the transverse magnetization associated with the excited nuclei relaxes back to its equilibrium value of zero. This relaxation produces a changing magnetic field which is measured in adjacent pickup coils. A representation of this relaxation is the FID curve.
The analysis method described herein and in the above related patents and applications is to decompose the FID waveform into a sum of separate time function equations. The coefficients of these equations are derived from the FID by use of a Marquardt-Levenberg (M-L) iterative approximation that minimizes the Chi-squared function--a technique well known in the art. Some of the time function equations found useful are: Gaussians, exponentials, Abragams (Gaussian) * (sin(t)) * (1/t) , modified Gaussian ((Gaussian) * (cos (sqrt (t))) *1/sqrt (t))) and trigonometric. From these time functions a set of parameters is calculated. Some of these parameters are ratios of the y-axis intercepts, squares and cross products of these ratios, and decay times for each of the time curves.
In addition the sample temperature may form the basis for another parameter. A limitation exists since any temperature difference between the sample and the sample chamber and the local environment will cause the sample temperature to change during the measurement. This changing temperature creates errors which may become significant. Such errors have been observed when measuring and then predicting the solids content in processed cheese, and, similarly, various flow rate measurements in polyolefins.
The first temperature measuring problem is the measuring probe which is inserted into the sample. But placing the probe disturbs the sample flow, possibly blocking or bridging the flow during filling and emptying, and the probe itself may cause the sample temperature to change. In short, errors will occur. Infrared temperature sensing techniques suffer from the need to place a window viewing the sample, allowing energy to escape through the window (in order to sense the temperature) and so causing errors.
Melt Index (MI) is an industry term for polyethylene defined as the flow rate obtained under Condition 190/2.16, as found in Note 19 of ASTM (American Society for testing and Materials) No. D1238-90b, and Flow Rate Ratio (FRR) is similarly defined for polyethylene as the dimensionless number derived by dividing the flow rate at condition 190/10 by the flow rate at condition 190/2.16, as found in Paragraph 8.3, page 396 of ASTM No. D1238-90b. In some instances, the logarithm of FRR is used in lieu of FRR. Melt Flow (MF) is an industry term for polypropylene defined as the flow rate at condition 230/2.16 of the ASTM No. D1238-90b. It should be noted that the above are not all universally accepted as indicated in above referenced Note 19. This note suggests, other than MI for polyethylene, only the use of flow rates for all materials regardless of the condition used.
Melt index (MI), flow rate ratio (FRR) and melt flow (MF) measurements are routinely done by technicians using manual or automated plastometers that are slow and tedious to use. There is a need to automate and develop on-line techniques fort the rapid and accurate measurements of MI, FRR and MF, as these are important parameters that are widely used for quality control purposes.
Section 4.3 of the above referenced ASTM D1238-90b specification publication states, "The flow rate obtained with the extrusion plastometer is not a fundamental polymer property. It is an empirically defined parameter critically influenced by the physical properties and molecular structure of the polymer and the conditions of measurement. The rheological characteristics of polymer melts depend on a number of variables. Since the values of these variables occurring in this test may differ substantially from those in large-scale processes, test results may not correlate directly with processing behavior." The plastics industry measures flow rates as described in the ASTM specification and infers the physical and chemical properties such as crystallinity, average molecular weight and the molecular weight distribution. Since the flow rates are not fundamental properties of plastics--the physical properties can only be inferred.
It is a principal object of this invention to measure physical properties of plastics and relate those properties back to flow rates by calibration with known samples.
Polyethylene, including linear polyethylene may be considered as having three major components, namely, a crystalline, an interfacial and an amorphous. For material with higher MIs, the crystalline region is efficiently packed providing less interfacial content (a parameter). In amorphous regions in higher MI samples, an extra degree of randomness results in greater free volumes. This means that methyl groups at the ends of the polymer chains or localized chain units involving 10-15 bonds undergo less restricted motion in material with higher MIs. Better averaging of dipole-dipole interaction (due to enhanced motion) therefore results in a longer time constant (a parameter) for decay of NMR signals originating from amorphous regions. Conversely, shorter time constants result in materials with lower MIs. Similar observations have been made for polypropylene and other plastics.
But, relating these previously mentioned parameters, quantitatively and qualitatively, back to the species of target nuclei is required. In the above referenced patent applications, the system is calibrated with known samples, and a regression equation is generated which relates the parameters to the types, properties and quantities of the target nuclei. An unknown sample is introduced and the time functions are derived via the M-L iteration, and the parameters are calculated. The parameters are "regressed" via the regression equation to yield the types, properties and quantities of target nuclei in the unknown sample. That is, the measured parameters from the unknown FID are used with the regression equation, and the types, properties and quantities in the unknown sample are determined. It is to be understood that the multidimensional regression equation may not be graphically represented, and that the regression equation may be non-linear. As a simple regression technique example, consider that the grade point average of each of the students at a college were related to that student's SAT score and high school standing (forming a three dimensional space). The line formed is a "regression line" (which may be graphed). A new student' s grade point average may be predicted by placing the student's SAT and high school standing on the "regression line" and "reading" the grade point average.
It is a principal object of the present invention to obtain flow rates for plastics, (melt index and flow rate ratio for polyethylene, and melt flow for polypropylene) via NMR techniques.
It is another object of this invention to compensate for temperature changes in the sample under test.
It is yet another object of this invention to relate the type, property and quantity of target nuclei of interest accurately and precisely.